This invention relates to electrochemical cells and batteries, and more particularly, to such cells and batteries having lithium-based active material.
Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells have included an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between electrically insulated, spaced apart positive and negative electrodes. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. During use of the cell, lithium ions (Li+) are transferred to the negative electrode on charging. During discharge, lithium ions (Li+) are transferred from the negative electrode (anode) to the positive electrode (cathode). Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Cells having metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemically active material of the cathode whereupon electrical energy is released. During charging, the flow of lithium ions is reversed and they are transferred from the positive electrode active material through the ion conducting electrolyte and then back to the lithium negative electrode.
The lithium metal anode has been replaced with a carbon anode, that is, a carbonaceous material, such as non-graphitic amorphous coke, graphitic carbon, or graphites, which are intercalation compounds. This presents a relatively advantageous and safer approach to rechargeable lithium as it replaces lithium metal with a material capable of reversibly intercalating lithium ions, thereby providing the so-called xe2x80x9crocking chairxe2x80x9d battery in which lithium ions xe2x80x9crockxe2x80x9d between the intercalation electrodes during the charging/discharging/recharging cycles. Such lithium metal free cells may thus be viewed as comprising two lithium ion intercalating (absorbing) electrode xe2x80x9cspongesxe2x80x9d separated by a lithium ion conducting electrolyte usually comprising a lithium salt dissolved in nonaqueous solvent or a mixture of such solvents. Numerous such electrolytes, salts, and solvents are known in the art. Such carbon anodes may be prelithiated prior to assembly within the cell having the cathode intercalation material.
In a battery or a cell utilizing a lithium-containing electrode it is important to eliminate as many impurities as possible which may affect cell performance. More particularly, the rechargeability of a lithium metal foil electrode is limited by side reactions between metallic lithium and impurities. When impurities react with lithium there is formed a solid surface layer on the lithium which increases the impedance of the anode (negative electrode). Non-metallic, carbon anodes are also subject to passivation through reaction with cell impurities.
Loss of performance due to impurities has lead to the selection of solvents and salts which are less reactive with cell components. Yet, this avoids use of some solvents and salts which would have better performance in a cell as compared to their less reactive counterparts. In another approach, as exemplified in U.S. Pat. No. 5,419,985, acidic desiccants, and/or hydrolyzable compounds are added to precursor components of the cell. These compounds are used to take up water or hydrolyze with water and then the hydrolysis products are removed before the cell components are assembled. However, since the source of impurities which causes adverse reaction may be from any component within the cell, including negative electrode, positive electrode, and electrolyte, it is very difficult to completely eliminate the impurities prior to assembly of the completed cell. Therefore, such desiccants and hydrolyzable compounds are not sufficiently effective. This is particularly evident since after assembly of the cell, moisture and other impurities from the environment may penetrate through the cell""s protective covering. Therefore, what is needed is an understanding of the mechanisms by which impurities cause undesirable loss of performance and reduce cycle life of battery due to undesirable interaction with impurities. Although interaction with metallic lithium has now been resolved by eliminating the use of the metallic lithium, yet there still remains the challenge of determining how impurities cause detrimental loss of capacity and an effective means for preventing loss of cell performance as a result of such interaction.
In one embodiment, the invention provides a novel composition and method for preventing decomposition of one or more electrochemical cell components comprising an electrode having an active material, and an electrolyte. In PCT/US97/22525 filed Nov. 21, 1997 and in then U.S. Ser. No. 08/762,081 filed Dec. 9, 1996, now U.S. Pat. No. 5,869,207, there is described a method which effectively overcomes problems which arise between the interaction of cell components and contaminate water retained in a cell. Such contaminate water reacts with the electrolyte which comprises a salt of lithium in a solvent. Solubilizing of the salt in solution with attendant interaction between the salt and water causes formation of hydrogen-containing acids. The method of the invention effectively blocks decomposition of a lithium metal oxide cathode active material, and particularly lithium manganese oxide (LMO, nominally LiMn2O4). Such decomposition is prevented by including in the cell a basic compound which forms an electron donor species in the electrolyte solution; and by neutralizing at least a portion of the acid by reacting the donor species with the hydrogen-containing acids thereby preventing decomposition of the lithium manganese oxide by the acid. The preservation of the lithium manganese oxide prevents degradation of other cell components by other mechanism. In the aforesaid applications, it was shown that subsequent additional related reactions occur to the same extent as the decomposition of the LMO, suggesting that the LMO break down provides a catalytic effect which causes one or more of the following: generation of water which in turn is capable of being reduced to hydrogen (H2) gas at the anode; generation of additional hydrogen-containing gas (HY, where Y is the anion, for example, HF); and generation of additional decomposition products from components in the cell such as the electrolyte solvent, forming any of a variety of gases such as carbon monoxide, carbon dioxide, and methane, which may further decompose to form H2. The evolution of hydrogen gas by reduction at the anode significantly increases to volumetric size of the battery. In one embodiment described in the aforesaid applications, the basic compound of the invention forms electron donor species by dissociation in solution when the basic compound is represented by MX where M represents a metal and X represents the electron donor species. In another mechanism, the basic compound additive is an organic compound which provides electron donor species, such as in the case of an NH2 group which is capable of forming an NH3 thereby interfering with formation of the acid component, with the result that acid attack of cell elements is prevented.
The electrochemical cell of the present invention contains LMO stabilized against decomposition. In one embodiment, the cell of the invention comprises the electrolyte, the lithium salt, and a solvent which solubilizes the salt. The cell comprises lithium manganese oxide (LMO) active material and a lithium-containing compound adjacent particles of the LMO active material, and desirably in intimate contact with the LMO active material. More desirably the lithium compound is dispersed on and carried on the LMO particle surface. In another embodiment, the lithium compound is at least partially decomposed in the presence of the LMO particle, causing the lithium content of the LMO to increase. More desirably, the lithium content of the LMO spinel is increased by essentially complete decomposition of the lithium compound. The embodiments described above are combined to optimize performance.
In the aforesaid applications, the basic compound additives are selected from the group consisting of carbonates; metal oxides; hydroxides; amines; organic bases, particularly those having up to 6 carbon atoms are desirable, such as alkyls and phenols, butylamines; aluminates; and silicates. Most preferred are lithium-based compounds, such as lithium carbonates, lithium metal oxide, lithium mixed metal oxides, lithium hydroxides, lithium aluminates, and lithium silicates. Here the preferred lithium compound is lithium carbonate which decomposes in the presence of LMO at a temperature in a range of 600xc2x0 C. to 750xc2x0 C., and as low as 400xc2x0 C.
In one embodiment, the invention provides a method of treating spinel lithium manganese oxide particles which comprises first forming a mixture of the lithium manganese oxide particles and lithium carbonate. Next, the mixture is heated for a time and at a temperature sufficient to decompose at least a portion of the lithium carbonate in the presence of a lithium manganese oxide. Depending on the temperature selected, a portion of the lithium carbonate is decomposed or reacted with the lithium manganese oxide and a portion of the lithium carbonate is dispersed on the surface of the lithium manganese oxide particles. The result is a treated spinel lithium manganese oxide characterized by reduced surface area and increased lithium content as compared to an untreated spinel lithium manganese oxide. In one alternative, essentially all of the lithium carbonate is decomposed or reacted with the lithium manganese oxide.
In one aspect, the heating is conducted in an air atmosphere or in a flowing air atmosphere. In one embodiment, the heating is conducted in at least two stages beginning at an elevated temperature. Heating is preferably conducted under at least two progressively lower temperatures followed by cooling to an ambient temperature. In one example, progressive stages of heating are conducted, a first stage is in a range of 650 to 700xc2x0 C., then at a lower temperature on the order of 600xc2x0 C., then at a lower temperature in a range of 400 to 500xc2x0 C., followed by permitting the product to cool to an ambient condition. Quenching is considered optional. The heating is conducted for a time up to about 10 hours and the amount of lithium carbonate contained in the mixture is about 0.1% to about 5% by weight of the total mixture.
The product of the aforesaid method is a composition comprising particles of spinel lithium manganese oxide (LMO) enriched with lithium by a decomposition product of lithium carbonate forming a part of each of the LMO particles; and the product is characterized by a reduced surface area and improved capacity retention with cycling, expressed in milliamp hours per gram, as compared to the initial, non-enriched spinel. In one aspect, the decomposition product is a reaction product of the LMO particles and the lithium carbonate. The lithium-rich spinel so prepared is represented by the formula Li1+xMn2xe2x88x92xO4 where x is greater than or equal to 0.08 and less than or equal to 0.20, preferably x is greater than 0.081. The character of the product is further defined below. This lithium-rich spinel product is preferably prepared from a starting material of the formula Li1+xMn2xe2x88x92xO4 where Oxe2x89xa6xxe2x89xa60.08, and preferably the starting material has x greater than 0.05. The lithium-rich spinel product has an Li content greater than that of the LMO starting material.
The product of the aforesaid method will depend upon the extent of heating during heat treatment. If all the lithium carbonate is decomposed or reacted, then the lithium enriched spinel is produced. If some of the lithium carbonate remains unreacted or not decomposed, then it is dispersed on and adhered to the surface of the lithium-rich spinel particles.
In still another embodiment, the heat treated spinel in particle form is mixed with lithium carbonate in particle form, and the particle mixture is used to form an electrode. The electrode comprises the particle mixture, a binder and, optionally, conductive material such as carbon powder.
Objects, features, and advantages of the invention include an improved electrochemical cell or battery based on lithium which has improved charging and discharging characteristics; a large discharge capacity; and which maintains its integrity over a prolonged life cycle as compared to presently used cells. Another object is to provide stabilized electrochemical cells which are stabilized against decomposition of cell components, including electrode and electrolyte components.